ln: enforce trampoline onion constraints

We add a `check_trampoline_constraints` similar to
`check_blinded_path_constraints` that compares the Trampoline onion's
amount and CLTV values to the limitations imposed by the outer onion.

Tests are added to cover validation of blinded and unblinded trampoline
payloads against their outer onion. These are consolidated with our
existing coverage for successful receives.

Co-authored-by: Arik Sosman <git@arik.io>
Co-authored-by: Maurice Poirrier <mpch@hey.com>

Author: 3 people